Automatic transmissions have now been implemented on automobiles for a number of decades. Their popularity is mainly due to the fact that with an automatic transmission the driver can pay more attention to the varying road conditions without the additional task of changing gears, and thereby improving the safety of the driver, the passengers and the other road users. By the same token, as cycling has become increasingly popular in recent years, there have been growing demands for automatic transmissions that can be used on bicycles.
In the prior art, a number of automatic bicycle transmissions can be found, and they can, in general, be divided into the three categories, namely fluid-drive, centrifugal-type and torque-responsive type.
The basic working principle of a fluid-drive transmission is to utilize the drag of a fluid contained within a chamber to transfer power. For example, in the fluid automatic bicycle transmission as disclosed in U.S. Pat. No. 7,059,618, a chamber is formed between an outer shell which is driven by an applied input torque (e.g. from the cyclist) and an inner shell which delivers output power to the rear wheel of the bicycle. Within the chamber, there contains a suitable fluid for power transfer between said outer and inner shells, and a mechanical vane stator assembly which, by changing the clearance between the vanes and the outer shell, can alter the drag within the fluid. The mechanical vane stator comprises a number of vanes which are each connected to a central stator via an individual spring or any elastic means. When the driving torque is increased, the outer shell, which is driven by the driving torque, applies through the fluid a larger force onto the vane stator assembly, and consequently the vanes are pushed towards the centre of the stator and away from the outer shell. As a result, a larger clearance between the vanes and the outer shell is being presented, and subsequently the slippage between the outer and inner shells is increased. This change in slippage is effectively a change in mechanical advantage between the input and output torque, and, in the situation described above, the change in mechanical advantage corresponds to a shift to a “lower gear” or a “downshift”. A similar but reverse operation is performed when the input torque is reduced. A fluid-drive transmission system as described above has a number of drawbacks. Firstly, the transmission does not have a ‘holding’ function which maintains the drive ratio permanently or temporarily when the input torque is being varied. This is a desirable function in a number of circumstances, for example the cyclist may want to avoid an automatic upshift when his/her input torque is briefly removed during an uphill climb. The second drawback is that since the whole transmission assembly is filled with the transmission fluid, it might be quite troublesome if the transmission system needs to be dismantled for maintenance or repair. Another drawback of the above fluid-drive system is that the existing derailleur, sprockets and shifters on a conventional, manual-transmission bicycle will have to be removed before the installation of such system, and so it cannot be easily retrofitted onto a conventional bicycle.
Regarding the centrifugal-type automatic transmissions, the basic working principle is to implement the gear-shifting operation by harnessing the power from the rotational motion of the bicycle wheels. More specifically, a particular component of the system is caused to rotate at a speed proportional to that of the bicycle wheels. The centrifugal force that is created from such rotation will then directly or indirectly induce the movement of the gear-selecting component of the system, and subsequently lead to a change in the drive ratio. An example of a centrifugal-type automatic bicycle transmission is disclosed in U.S. patent application Ser. No. 11/670,570. In that patent application, a transmission system which comprises a rotatable shaft, a plurality of centrifugal weights pivotally connected to each other and to a star-shaped collar, a collar assembly that includes the above star-shaped collar and a rear derailleur is disclosed. The rotational motion of the bicycle wheels causes the centrifugal weights to rotate at a directly-proportional speed. As the rotational speed of the centrifugal weights increases, the increased centrifugal force would cause the centrifugal weights to flare outwardly by an increasing amount. As the centrifugal weights flare outwardly, the collar assembly is then forced to move along the rotatable shaft, and consequently the chain guide of the rear derailleur would move outwardly to derail the drive chain from a larger diameter sprocket to an adjacent smaller diameter sprocket, thereby resulting in an “upshift” to a higher gear as the speed of the bicycle increases. The collar assembly is biased such that the reverse of the above operations would occur as the speed of the bicycle decreases. A direct result of this design is that whenever the bicycle is at rest, the largest diameter sprocket (i.e. the lowest gear) will be automatically selected. This is an undesirable feature because when a low gear is selected a high pedaling cadence is needed to accelerate the bicycle. However, the design of the centrifugal-type transmission is such that upshifting will not occur unless a certain travelling speed is attained, and therefore it may actually be more tiring to cycle with such centrifugal-type automatic transmission than with a conventional manual transmission. Furthermore, the installation of this centrifugal-type transmission requires modifications to the existing rear sprocket assembly, and so it could be a difficult operation to retrofit such system onto a conventional bicycle.
A third type of automatic bicycle transmission is the torque-responsive design, and an example of which is disclosed in U.S. Pat. No. 5,061,224. The transmission system that is disclosed therein comprises a variable-pitch pulley consisting of two opposite-side, slidable sheave members and a V-belt which connects the variable-pitch pulley to a driven pulley. The variable-pitch pulley is sensitive to the torque applied by the cyclist to the bicycle pedal. As the applied torque increases, the distance between the variable-pitch pulley and the driven pulley extends, and so the V-belt is forced to move inwards towards the middle of the two sheave members where the radius of curvature is smaller. As a result, a reduced-diameter drive pulley is now driving the driven pulley, and therefore, a “downshift” has been effectively performed. A major drawback of this invention is that whenever the applied torque is removed, for example during coasting, the gear ratio will immediately return to a “low torque, high speed” configuration. This can cause inconvenience to the cyclist especially when he/she wishes to take a brief rest to conserve energy while cycling up an incline.
Another torque-responsive automatic bicycle transmission which addresses the above problem is disclosed in U.S. Pat. No. 4,781,663. An embodiment of that invention comprises a drive pulley and a driven pulley connected via a V-belt. Both the drive pulley and the driven pulley are made up of a plurality of retractable arms such that the effective diameters of the pulleys can be varied by the expansion or retraction of those arms. Upon application of an increased torque, the tension within the V-belt will cause, via a number of components, the drive pulley arms to retract to form a reduced-diameter pulley. Meanwhile, the tension within the belt will ensure that the arms of the driven pulley shall expand accordingly. As a result, the gear ratio is adjusted to provide a higher output torque. When the input torque is reduced, the drive pulley and the driven pulley will expand and contract respectively using the same mechanism as described above. This invention further includes a “hold system” which prevents the gear ratio from changing even when the applied input torque has exceeded or reduced to a level at which the gear ratio would normally change. This holding function is accomplished by applying an axial force to the opposite surfaces of the pulley arms to hold them in their respective position through frictional engagement of the opposite arm surfaces and two side cam plates. This holding function is desirable, but to perform such function purely by frictional engagement may not be the best method since the applied torque could be large enough to overcome the “holding” friction. Moreover, wear and tear may occur as a result of any slippage between the frictionally-engaged arm surfaces and the side cams, and so the effectiveness of the hold system may deteriorate after a certain period of use. Furthermore, the hold system would only operate if it has been manually activated by the cyclist.
A third example of a torque-responsive type automatic bicycle transmission is disclosed in U.S. Pat. No. 3,769,848. A preferred embodiment of the disclosed invention comprises a heavily modified rear sprocket assembly slidably installed onto a rear axle designed to replace that on a conventional bicycle. The rear axle comprises a shaft on which a spiral track and a number of recesses, formed parallel to the track, are provided around its periphery. Installed onto the axle alongside the rear sprocket assembly is a free-wheeling circular member which contains two hardened balls. One of them is to be received by the track while the other is to be received by one of the recesses, exactly which depends on the rear sprocket that is being selected. When the input torque is increased, the circular member would rotate relative to the shaft, and consequently one of the hardened balls would travel along the spiral track, simultaneously forcing the other hardened ball to move from one recess to the next closest one and thereby displacing the rear sprocket assembly along the rear axle such that the next largest rear sprocket will be engaged via a conventional bicycle derailleur. When the input torque is reduced, a spring provided on the rear axle would urge the rear sprocket assembly, through the ball-and-track mechanism, to move in the direction such that the next smallest rear sprocket is engaged. Several additional features have also been disclosed that may be incorporated into the previously described embodiment. The first of which is a dash-pot contained within the shaft for preventing undesired shifting in response to momentary changes in the applied torque. Another additional feature is a locking mechanism consisting of a locking plate and a set of grooves formed on an enlarged portion of the shaft, together they prevent any movement of the rear sprocket assembly relative to the axle and thereby inhibiting any change in the rear gear ratio even when the applied torque is being varied. A third additional feature that was disclosed is a simplified locking mechanism comprising a detent and a receiving slot. If the automatic shifting function is to be deactivated, the detent can be switched to an operative position at which the rear sprocket assembly will be inhibited from moving along the axle, and hence deactivating the automatic shifting function. To reactivate the automatic shifting function, the detent can be switched back to its original position. A drawback with this driving mechanism is that it does not allow the automatic shifting function to be only temporarily deactivated, i.e. the mechanism can only be activated when the smallest diameter sprocket is being selected. Moreover, another drawback of this invention is that its installation requires a large amount of modifications to be made to a conventional bicycle.
In addition, all of the above prior art inventions would only adjust the gear or gear ratio of the rear sprocket assembly while that of the front sprocket assembly will remain unchanged. However, if the gear of the front sprocket assembly can also be shifted, the change in the mechanical advantage of the whole transmission system can be carried out more effectively and the system can also be utilized to a greater extent. Furthermore, all of the above “hold” systems would have to be activated manually, which would present an extra distraction to the cyclist.
Therefore, there is a need in the prior art for an automatic bicycle transmission by which the gears of both the front and rear sprocket assemblies can be shifted. It is highly desirable that such transmission system can automatically “hold” any selected gear ratio even when the applied input torque is being varied, and such hold system should be durable. Furthermore, the transmission system should automatically select the highest gear when starting from rest, or at least the cyclist should be able to maintain a comfortable pedaling cadence at all speeds. Additionally, it should be easy to retrofit the automatic transmission system onto a conventional bicycle, as well as to maintain and repair it afterwards.